Paper products are normally shipped in rolls from a paper mill to a converting or printing facility. Rolls made from different paper machines or made at different times or locations of the machine may have different reel building and roll runnability characteristics, where “runnability” is an indication of how well the roll pulls through the paper-making, converting, and printing processes, as well as the flatness and uniformity of the resulting web of paper.
Online paper finishing with multi-nip calenders is well known for building reels of super-calendered (SC) or light-weight coated (LWC) paper. High quality printing papers that are calendered online are thin, very dense and therefore resistant to additional compression. With these paper properties, it has-been found that traditional methods of cross direction (CD) reel build control using contacting calliper (thickness) sensors have been difficult to optimize. In order to precisely build an SC reel with good runnability, it is known in the art to monitor and control multiple properties during the manufacturing process, such as dry weight, moisture, and caliper (thickness). More particularly, these (and other) sheet properties may be controlled in a sheet-making machine in order for the sheet properties to match, as closely as possible, predefined target or desired values.
The control of sheet properties is accomplished through the use of various actuators, such as machine direction (MD) actuators that control the cross direction average of a sheet property, and cross direction (CD) actuators that affect both the average of a sheet property and the cross direction shape of the sheet property. In general, the cross direction (CD) is typically perpendicular to the machine direction (MD). Overall control of sheet properties presents a problem of very large scale, with multiple inputs and outputs (e.g. several hundred CD actuators may be required to control one or more paper quality profile(s) consisting of typically 500-1200 measurement points each corresponding to 5-10 mm resolution across the web). To that end, multivariable control processes have been developed for cross-direction paper quality control, as set forth, for example, in US Patent Publication 2008/0017341 (Maenpaa et al); Calvin Fu, Jarmo Ollanketo and Jukka Makinen, “Multivariable CD Control and Tools for Control Performance Improvement”, published at the Control Systems 2006 Conference, Jun. 6-8, 2006, Tampere, Finladd, pp. 215-220; and Seyhan Nuyan, Calvin Fu and Steven Bale, “CD Response Detection for Control”, presented in 1998 at the TAPPI PCE&I Conference, Vancouver, BC, Mar. 16-29, 1998, pp. 95-105 (hereinafter “Nuyan et al”).
There remains a significant challenge in determining which of a multitude of paper quality profiles (e.g. reel diameter, hardness; pre-wound or wound-in tension, moisture, caliper (thickness), etc.) should be selected as control variables in a multivariable CD (MVCD) control process to address different problems (e.g. degraded roll runnability due to air entrapment versus mass variations in the web). The challenge in selecting appropriate control variables or profiles is particularly acute with highly finished grades (i.e. highly calendered) which, as discussed above, are very thin, very dense, and are characterized by very low compressibility. For example, when highly calendered papers are wound in a reel, air accumulation between layers becomes a significant factor resulting in undesired reel diameter profile shape and abnormal reel building even if the caliper profile is flat or shaped to a desired target. Therefore, using only the caliper profile for CD control is not sufficient. On the other hand, simple reel diameter control also is not an adequate solution to reel building/roll runnability problems because reel diameter measurements alone do not distinguish between irregularities caused by air entrapment and mass (caliper). Moreover, conventional solutions to these two problems are mutually exclusive; i.e. correcting problems due to air-entrapment requires an opposite control action to the action required to correct problems caused by uneven caliper (mass).
As indicated above, hardness of the reel may also provide an indicator of the reel-build process. Reel hardness is traditionally measured as the amplitude of a pulse produced by a force button on a rotating wheel that contacts the paper web. The amplitude is correlated with the force or hardness of the reel, which may therefore be considered to represent a composite measurement that better describes the reel building process than caliper does. Nonetheless, hardness measurement alone also fails to provide sufficient information for adequately controlling the reel building process, for the reasons set forth above in connection with reel diameter and caliper.
Indeed, other complex interrelations may also exist between various factors that give rise to a particular problem (e.g. the effect of forces resulting from local tension variability (LTV) on air dynamics, correlations between local hardness and LTV, or correlations between tension and moisture profiles and hardness measurement).